The Café Wall Illusion is so simple, it's also called the Kindergarten Illusion. It's a combination of color, line, and tiling, and it completely screws up people's spatial reasoning.

Plenty of people probably knew about this illusion before, but the credit for officially describing it goes to Richard Gregory, a psychologist in the 1970s. A member of his staff, clearly bored, noticed that a nearby café wall had an alternating tile configuration that made the lines between the tiles look crazily tilted. Since it's actually very hard to make irregular tiling and relatively easy to use blocks and parallel lines — and since everyone in the office was probably bored — they investigated further and found that all the lines on the wall were perfectly parallel to each other. They just appeared tilted at strange angles.

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This is the Café Wall Illusion. When alternating bricks of black and white are stacked in a pattern any more ambiguous than a checkerboard, the lines between them seem tilted towards each other. What makes it work? We're not sure. But we know what makes it not work.

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If the bricks are done in colors of equal brightness, the illusion breaks down entirely. So, if the black and white tiles shown were to be swapped for strong shades of red and blue, we wouldn't see any tilt. That's not to say it doesn't work with colored tiles — just that it works only when bright and dark are contrasted. Also, the difference between the top set of bricks and the bottom, on the illusion shown to the left, is the color of the "mortar" between them. If the lines between them are gray, the lines appear far more tilted than they do if the lines are black.

The best guess anyone has for why this illusion works is the structure of the light and dark receptors on the retina. They think, perhaps, there is some bleed when light and dark are so close together, so that the eye sees some light areas as dark and some dark areas as light. Your brain then plots the line between the light and dark, and the bleeds cause it to plot the line as tilted.